Blackphone is sold unlocked and will work with any compatible GSM provider. In the US these include T-Mobile and AT&T but not Verizon or Sprint. For other countries we recommend checking with your carrier for what technologies they support. You may also find https://support.blackphone.ch/customer/portal/articles/1574842-specs and http://www.worldtimezone.com/gsm.html interesting.
An unlocked phone is one which is not restricted to only a single carrier's network. You own the phone, you have no commitment to any carrier. You can use it with any GSM carrier, or you can choose not to use it with a carrier at all, and only use it on Wi-Fi. Unlocked phones are useful if you want to shop around for different carriers based on price, as well as when you travel, since you can put any micro SIM card in your phone and have a local number on any other network.